Useful articles from various thermoplastic resins have been prepared from molds for many years. Polyarylene sulfide resins are among such moldable compositions and have been prepared, for example, as in U.S. Patent No. 3,354,129. A relatively recent development has been the use of polyarylene sulfide compositions such as, for example, polyphenylene sulfide compositions, for molding electronic components and as encapsulating materials for electronic components. These compositions typically represent a carefully balanced blend of at least polyarylene sulfide, glass fibers, and filler such as silica or talc. In addition, relatively small amounts of other components such as, for example, colorants, flow improvers, processing aids, organosilanes and like are typically present. Electronic components are encapsulated to maintain electrical insulation, to provide mechanical protection and to otherwise shield the component from exposure to its environment. As the evolution of electronics continues its rapid advance it becomes increasingly important that the art and technology of encapsulation keep pace. An area of significant concern and interest addressed by the present invention relates specifically to polyarylene sulfide compositions used to make molded electronic parts and to encapsulate electronic components.
Polyarylene sulfide, in particular, polyphenylene sulfide compositions are used to form electronic components and often to encapsulate electronic components in accordance with any encapsulation method suitable for thermoplastic encapsulation compositions. Such methods are well known in the art. One particularly useful method involves introducing the polyarylene sulfide composition into an injection molding apparatus to produce a melt which is extruded into an injection mold wherein the melt is shaped or wherein the electronic component to be encapsulated is positioned. Injection molding provides a convenient way for preparing various articles from polyarylene sulfide, particularly objects of a relatively intricate nature, e.g., electronic components.
In order to injection mold articles in an economic manner, the mold resident cycle time should be kept to a minimum. The shorter cycle time provides a shorter resin heat period with consequently less thermal damage to the resin and/or less thermal promoted interaction between the resin and various additives present in the resin.
In order to accomplish a release of the resin from the mold, various mold release agents have been found which provide for a release of the resin with lower ejection pressure. Mold release agents to be effective should be chemically compatible with the resin as measured by the usual characteristics of the resin under normal conditions and heat treatments.
Mold release of a thermoplastic resin from the mold is generally accomplished by the addition of a mold release spray applied to the mold to provide release of the part from the mold without sticking under standard molding conditions. Alternatively, the thermoplastic resin can be formulated with an internal lubricant which provides inherent mold release properties to the resin. There are many different internal mold release agents in practical use. However, these mold release agents are sometimes disadvantageous. For example, some commercial mold release agents highly effective for improving the mold release property of a thermoplastic resin decrease the mechanical strength of the molded article or discolor the molded article. Other mold release agents which do not degrade the mechanical strength and appearance of the mold article, do not satisfactorily improve the mold release property. Moreover, with respect to the molding or encapsulation of electronic components, it is important that the mold release agent and, generally, any additive, which is incorporated into the thermoplastic resin not add any ionic impurities such as Na, K, Li, Fe, Cl, etc. to the compounded product. There is a continuing need, therefore, to find useful internal lubricants to provide mold release properties to thermoplastic molding compositions. The present invention is directed to finding a useful internal lubricant for polyarylene sulfide.
One particular commercially employed mold release agent for thermoplastic resins, in general, is pentaerythritol tetrastearate. Several U.S. patents have been granted to the use of pentaerythritol tetrastearate as a mold release agent for specific thermoplastic resins.
U.S. Pat. No. 4,408,000 discloses a composition comprising a thermoplastic resin including among others polysulphones, polyethersulphones and polysulfides, a mold release agent such as PETS, and a mold release enhancing effective amount of a fatty acid. With regard to polycarbonate resins, it was found that a combination of PETS and a mixture of fatty acids showed greater mold release than an aromatic polycarbonate containing PETS alone as a mold release agent.
U.S. Pat. No. 4,409,351 illustrates that a fatty acid provides greater mold release to an aromatic polycarbonate than does PETS.
U.S. Pat. No. 4,530,953 utilizes PETS as a mold release agent for polyesters such as polyethylene terephthalate resin.